Radar sensors are used as distance sensors in motor vehicles, for example, and typically operate at a frequency of 77 GHz. For some time now, so-called MMICs (microwave monolithic integrated circuit) are used as transmission modules to generate the high frequency of 77 GHz or to convert it into signals that can be analyzed.
Control loops with high quality reference oscillators are frequently used to stabilize the signals. These reference oscillators convert the actual radar signal into a control frequency range suitable for a phase locked loop (PLL), and themselves normally operate at frequencies (e.g., 18 GHz) that may not be transmitted, or transmitted only at very low levels, due to relevant standards and regulations. For that reason the affected circuit elements are provided with shield casings.
On the other hand, however, the signal stabilizer must be linked to the transmission module in order to enable it to act on the latter's signals.
It is known to connect the shielded circuit elements via high-frequency lines installed on the circuit board, which requires corresponding openings in the shield casing. Casing channels are utilized, which function as hollow conductor filter structures to suppress radiation of the frequency components to be shielded through these openings.